Excavations such as mining operations, milling operations, road constructions, etc., generate effluents which may require treatment prior to discharge. These effluents include, for example, acid mine drainage (AMD), mill tailings, excess decant water, seepages, and acidic process waste streams. Acid mine drainage (AMD) forms when minerals in rocks are exposed to oxidizing conditions in mining operations, highway construction, and other large scale excavations.
Prior art methods for treating excavation effluents generally require extensive capital outlay and are specific in application to a particular effluent. In one example, lime neutralization to pH around 11 is used to remove a majority of contaminants. However, lime neutralization does not work with some metals such as antimony, vanadium, arsenic and molybdenum, and does not work well with anions such as sulfates, fluorides, nitrates and chlorides. In other examples, technologies developed for sulfate reduction may work to remove certain anions to government-mandated levels together with anionic species such as fluorides. However, they do not particularly work well with molybdenum and require 2-stage clarification/separation steps to isolate precipitates that interfere with downstream metal removal. Desalination technologies, e.g., membrane processes using reverse osmosis and ion-exchange, may work in removing molybdenum. However, they are expensive technology options and are prone to scaling issues due to calcium sulfate precipitation.
There is a need for improved methods for treating excavation effluents, particularly effluents with concentrations of anions such as fluorides, sulfates, molybdates, arsenates etc., as well as heavy metals such as nickel, cobalt, manganese, chromium, and the like.